The present invention relates to a display apparatus and, more particularly, to a display apparatus in which light from a display is directed toward a pupil through a diffraction grating such as a hologram to allow observation of an image displayed on a display.
Head-up displays for displaying an image from a display element through a hologram to be superposed on a forward visual field are conventionally known. Of these displays, in a head-up display apparatus for performing a display using a fluorescent display tube for emitting display light having a wide wavelength range, blurring of an image in the vertical direction caused by the wide wavelength range of the display light must be prevented.
Blurring caused in a display image becomes particularly conspicuous when the display image is displayed at a distance. If a half-width of a hologram wavelength characteristic (i.e. Diffraction Efficiency versus wavelength) is about 10 nm, degradation in image quality is visually observed.
As a means for correcting color dispersion caused by a hologram, Japanese Patent Publication No. 51-5956 (U.S. Pat. No. 3,580,655) discloses a technique for arranging an optical element (e.g., another hologram) for causing color dispersion opposite to that caused by the hologram in an optical path extending to the hologram.
U.S. Pat. No. 4,613,200 discloses an example in which this means is applied to the above-mentioned head-up display, and FIG. 1 shows an apparatus of that U.S. patent.
In the apparatus shown in FIG. 1, a pair of holograms 11 and 35 having a relatively large half-wavelength-width and an identical structure are arranged so that their reflecting surfaces are parallel to each other, thereby directing light from a display (not shown) toward a pupil 51 of an observer.
This apparatus can direct light from the display over the entire visible light region of red, green, and blue toward the pupil 51 without causing the blurring. However, as shown in FIG. 1, it is not easy to arrange the holograms 11 and 35 in a vehicle or the like in a parallel state. In this case, these holograms may be arranged in a non-parallel state to be housed in a narrow space of a cabin of a vehicle. However, the following problem is posed.
In a cabin of a vehicle, the pupil 51 of an observer is set at a position separated by about 800 mm from the position of a front window glass. However, the position of the pupil 51 largely varies depending on a driver's sitting height, movement of a seat in the back-and-forth direction, and the like.
FIG. 2 shows a state wherein wavelengths yielding maximum diffraction efficiencies of holograms 11 and 35 have a difference since the holograms 11 and 35 are arranged in a non-parallel state and the pupil 51 is largely displaced in the vertical direction. As described above, since the holograms 11 and 35 have the same characteristics, the half-widths of their wavelength characteristics are equal to each other. In FIG. 2, in a combination of two holograms (diffraction gratings) each having a half-wavelength-width of 20 nm, the center wavelengths of the holograms have a difference of 5 nm with respect to the pupil which is displaced in the vertical direction.
The amount of display light incident on a pupil is proportional to the product of the diffraction efficiencies of the two holograms. When the difference of 5 nm occurs, the amount of light is decreased by about 35%. To our surprise, the difference of 5 nm is caused by displacement of the pupil by only 30 mm in the vertical direction.
The offset of the center wavelengths appears as unevenness of brightness in a display screen when the pupil 51 is moved in the back-and-forth direction, thus degrading display quality.
Such a problem is conspicuously caused when the half-wavelength-width of the hologram is decreased to be smaller than that of light from a display in order to observe a bright forward visual field.
Therefore, if a conventional display apparatus using the diffraction gratings, e.g., holograms, is mounted in a vehicle or the like, an image on a display cannot always be clearly observed.